Truck drivers are blocking a key border crossing between the US and Canada, sparking fears of economic disruption.
While limited US-bound traffic is being allowed to cross the Ambassador Bridge in Ontario, Canada-bound lanes from Detroit remain closed.
Business associations have called for the bridge to be immediately cleared to ensure the steady flow of goods.
The protests across Canada against vaccine rules and Covid restrictions are now two weeks old.
Truckers are demonstrating against a rule that requires them to be vaccinated to cross the US-Canada border. But the protests have grown to include anger at restrictions and at Prime Minister Justin Trudeau’s government generally.
They have mostly centred on the capital, Ottawa, and another border crossing between Montana and Alberta has also been blocked.
The closure of the Ambassador Bridge is particularly significant because nearly 30% of annual trade between the US and Canada comes through it.
“I’ve already heard from automakers and food grocers,” Canadian Transport Minister Omar Alghabra said on Tuesday. “This is really a serious cause for concern”.
On Wednesday, White House press secretary Jen Psaki said that the White House was watching the situation “very closely”. She noted that authorities were tracking potential disruptions that the blockade could cause to agricultural exports from Michigan to Canada and the movement of workers back and forth across the border.
Lorry drivers rallying in solidarity with the demonstrators in Ottawa first blocked the bridge on Monday afternoon.
While the bridge was reopened to limited traffic from Canada into the US by Tuesday, authorities in Michigan have said that traffic flows into Canada remain blocked.
They continue to advise drivers to divert to nearby Port Huron to cross the border.
“The Windsor Police Service wants to resolve this situation through the use of diplomacy and negotiation, seeking a solution that is safe and sustainable for our community,” the police force said in a statement.
On Tuesday night, dozens of business associations based on both sides of the border called for a swift re-opening of the bridge, as well as the Alberta-Montana crossing.
“The disruption of the Ambassador Bridge is an attack on the well-being of our citizens and the businesses that employ them,” a statement from the organisations said.
“As our economies emerge from the impacts of the pandemic, we cannot allow any group to undermine the cross-border trade that supports families on both sides of the border.”
Late on Tuesday, the Royal Canadian Mounted Police (RCMP) said that protesters had also blocked the border crossing between Alberta and Montana for the second time in less than a week.
Over the weekend, the RCMP said that while most of the protesters have been respectful of officers and followed their directions, a number of investigations have been opened in relation to the blockade at the Alberta border.
In Alberta, the province’s proof-of-vaccination system expires on Wednesday at midnight. Additionally, from next Monday the province will drop a requirement that students wear masks in schools. Children under 12 will not be required to wear masks anywhere.